Method and apparatus for generating time-division multiplexed encoded transmission signal

ABSTRACT

The present invention provides a method of generating time-division multiplexed encoded transmission signals, including encoding optical pulse signals for each of a plural multiplexed channels and generating a transmission signal for each channel, performing time division multiplexing on first and second transmission signals and generating a 2-channel multiplexed signal modulating the multiplexed signal with a modulation signal having a frequency of (F−Δf) Hz, detecting a strength of a Δf Hz frequency component of the multiplexed signal changing a time delay amount of the second transmission signal with respect to the first transmission signal, and determining a time delay amount at which a strength of the Δf Hz frequency component is minimized and adjusting the transmission signals of the individual channels such that they are arranged at equidistant intervals on a time axis.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2009-038877 filed on Feb. 23, 2009 thedisclosure of which is incorporated by reference herein.

BACKGROUND

1. Technical Field

The present invention relates to a method of generating a time-divisionmultiplexed encoded transmission signal that is used in a communicationsystem where an optical time-division multiplexing method and an opticalcode-division multiplexing method are used together, and an apparatusfor realizing the same.

2. Related Art

In recent years, with the development of the Internet, communicationdemands have rapidly increased. For this reason, a communicationcapacity has increased. As an example of a method for increasing thecommunication capacity, optical multiplexing technologies forcollectively transmitting optical pulse signals corresponding to pluralchannels through one optical fiber transmission path have attractedattention.

As the optical multiplexing technologies, an optical time-divisionmultiplexing (OFDM) scheme, a wavelength-division multiplexing (WDM)scheme, and an optical code division multiplexing (OCDM) scheme havebeen actively studied.

According to the optical multiplexing technologies, optical pulsesignals corresponding to plural channels may be collectively transmittedthrough one optical fiber transmission path. Accordingly, thecommunication capacity may be increased while the existing communicationnetwork is maintained as it is. If these optical multiplexingtechnologies are combined, the communication capacity may be furtherincreased.

Accordingly, a communication system where the OTDM scheme and the OCDMscheme are used together is suggested (for example, refer to “Study OfOTDM Channel Identifying Technique Using FBG Type Optical Encoder” inRef. 1 Proceedings of the 2007 Communications Society Conference of theInstitute of Electronics, Information and Communication Engineers,B-10-60).

In the communication system where the OTDM scheme and the OCDM schemeare used together, in order to efficiently realize the OTDM scheme, thetime delay amount of an encoded transmission signal of each channelneeds to be adjusted, such that the encoded signals of the individualchannels are arranged at an equivalent interval on a time axis. In termsof effective utilization of communication resources, a time slot that isallocated to each channel may be minimized.

If the time slot that is allocated to each channel is minimally set,adjacent channels may overlap each other due to a change in acommunication state. If the channels overlap, crosstalk may be generatedbetween the adjacent channels. If the time slot is secured to beexcessively wide, the time slot includes an unnecessary time zone,although the adjacent channels do not overlap each other. Accordingly,this becomes an obstacle when communication resources are effectivelyused and communication efficiency is improved.

In order to minimize the time slot and prevent the crosstalk from beinggenerated between the adjacent channels, such that the encodedtransmission signals of the individual channels may be dynamicallyadjusted so that they are arranged at an equivalent interval on the timeaxis. The optimal positions of the encoded transmission signals of theindividual channels on the time axis are momentarily changed accordingto a communication state.

Accordingly, in the communication system where the OTDM scheme and theOCDM scheme are used together, the time delay added to the encodedtransmission signals of the individual channels need to be flexiblychanged according to the momentarily changed communication state, suchthat the encoded transmission signals of the individual channels arearranged at the equivalent interval on the time axis.

However, in the communication system where the OTDM scheme and the OCDMscheme are used together, a method that adjusts the encoded transmissionsignals of the individual channels to be arranged at the equivalentinterval on the time axis has not been proposed.

SUMMARY

Accordingly, the present invention provides a method that adjustsencoded transmission signals of individual channels to be arranged at anequivalent interval on a time axis, and an apparatus for realizing thesame.

An OTDM/OCDM transmission signal generating method according to anaspect of the invention is a method of generating time-divisionmultiplexed encoded transmission signals, including: encoding opticalpulse signals for each of a plural multiplexed channels whose bit rateis F bit/s and generating a transmission signal for each channel;performing time division multiplexing on first and second transmissionsignals selected from the transmission signals of the plural multiplexedchannels and generating a 2-channel multiplexed signal; modulating themultiplexed signal with a modulation signal having a frequency of (F−Δf)Hz, where Δf is defined as a real number satisfying the condition Δf>0and F is defined as a real number satisfying the condition F>Δf;detecting a strength of a Δf Hz frequency component of the multiplexedsignal modulated by the modulation signal; changing a time delay amountof the second transmission signal with respect to the first transmissionsignal, and determining a time delay amount at which a strength of theΔf Hz frequency component is minimized; and applying a time delay to thetransmission signal of each of the multiplexed channels on the basis ofthe determined time delay amount, and adjusting the transmission signalsof the individual channels such that they are arranged at equidistantintervals on a time axis.

The OTDM/OCDM transmission signal generating method according to theaspect of the invention is realized by an OTDM/OCDM transmission signalgenerating apparatus according to another aspect of the inventiondescribed below.

The OTDM/OCDM transmission signal generating apparatus according toanother aspect of the invention is an apparatus for generatingtime-division multiplexed encoded transmission signals, including:encoded transmission signal generators that encode optical pulse signalswhose bit rate is F bit/s and output generated transmission signals, thenumber of the encoded transmission signal generators corresponding tothe number of a plural multiplexed channels; an optical multiplexer thatperforms time division multiplexing on first and second transmissionsignals output from two encoded transmission signal generators selectedfrom the encoded transmission signal generators, and generates a2-channel multiplexed signal; a spectrum analyzer that modulates themultiplex signal by a modulation signal having a frequency (F−Δf) Hz,where Δf is defined as a real number satisfying the condition Δf>0 and Fis defined as a real number satisfying the condition F>Δf, and detects astrength of a Δf Hz frequency component in the multiplexed signalmodulated by the modulation signal; and a optical delay amountcontroller that changes the time delay amount of the second transmissionsignal with respect to the first transmission signal, determines thetime delay amount Δt which the strength of the Δf MHz frequencycomponent is minimized, sets the time delay amount of the transmissionsignal of each of the multiplexed channels on the basis of thedetermined time delay amount Δt, and adjusts the transmission signals ofthe individual channels such that they are arranged at equidistantintervals on a time axis.

If the OTDM/OCDM transmission signal generating method according to theaspect of the invention is constructed as a method of generating2-channel OTDM/OCDM transmission signals, the OTDM/OCDM transmissionsignal generating method becomes an OTDM/OCDM transmission signalgenerating method described below.

The OTDM/OCDM transmission signal generating method according to anotheraspect of the invention is a method of generating time-divisionmultiplexed encoded transmission signals, including: encoding an opticalpulse signal of a first channel whose bit rate is F bit/s and generatingand outputting a first transmission signal; encoding an optical pulsesignal of a second channel whose bit rate is F bit/s and generating andoutputting a second transmission signal; inputting the secondtransmission signal and applying a time delay to the second transmissionsignal; performing time division multiplexing on the first transmissionsignal and the second transmission signal to which the time delay hasbeen applied, and generating a 2-channel multiplexed signal; modulatingthe multiplexed signal with a modulation signal having a frequency(F−Δf) Hz, where Δf is defined as a real number satisfying the conditionΔf>0 and F is defined as a real number satisfying the condition F>Δf,and generating a modulated multiplexed signal; converting the modulatedmultiplexed signal into an modulated multiplexed electrical signal andoutputting the modulated multiplexed electrical signal; changing thetime delay amount applied to the second transmission signal, anddetermining a time delay amount at which the strength of Δf Hz frequencycomponent of the modulated multiplexed electrical signal is minimized;and setting the time delay of the second encoded transmission signal onthe basis of the determined time delay amount, and adjusting the firstand second encoded transmission signals such that they are arranged atequidistant intervals on a time axis.

The OTDM/OCDM transmission signal generating method that generates the2-channel OTDM/OCDM transmission signals is realized by an OTDM/OCDMtransmission signal generating apparatus described below.

The OTDM/OCDM transmission signal generating apparatus according toanother aspect of the invention is an apparatus for generatingtime-division multiplexed encoded transmission signals, including: firstand second transmission signal generators that encode optical pulsesignals whose bit rate is F bit/s, and generate and output generatedtransmission signals; an optical delayer disposed in the secondtransmission signal generator that applies a time delay to atransmission signal output from the second transmission signalgenerator; an optical multiplexer that performs time divisionmultiplexing on the first transmission signal and the secondtransmission signal to which the time delay has been applied, andoutputs a 2-channel multiplexed signal; an optical branching device thatbranches the multiplexed signal into a multiplexed signal fortransmission and a multiplexed signal for monitoring; an opticalmodulator that receives the multiplex signal for monitoring, modulatesthe multiplexed signal for monitoring with a modulation signal whosefrequency is (F−Δf) Hz, where Δf is defined as a real number satisfyingthe condition Δf>0 and F is defined as a real number satisfying thecondition F>Δf, and generates a modulated multiplexed signal; aphotoelectric converter that receives the modulated multiplexed signal,converts the modulated multiplexed signal into a modulated multiplexedelectrical signal, and outputs the modulated multiplexed electricalsignal; a spectrum analyzer that detects the strength of a Δf Hzfrequency component of the modulated multiplex electrical signal; and anoptical delay amount controller that sets the optical delayer to thetime delay amount to apply a time delay at which the strength of a Δf Hzfrequency component is minimized to the second transmission signal, andadjusts the transmission signals of the multiplexed channels such thatthey are arranged at equidistant intervals on a time axis.

If the OTDM/OCDM transmission signal generating method according to theinvention is constructed as a method of generating 2^(N)-channelOTDM/OCDM transmission signals, the OTDM/OCDM transmission signalgenerating method becomes an OTDM/OCDM transmission signal generatingmethod described below.

The OTDM/OCDM transmission signal generating method according to anotheraspect of the invention is a method of generating time-divisionmultiplexed encoded transmission signals, including: encoding opticalpulse signals of 2^(N) channels (N is an integer of 1 or more) whose bitrate is F bit/s, and generating transmission signals; inputting a secondtransmission signal corresponding to one transmission signal of firstand second transmission signals selected from the transmission signalsof the 2^(N) channels, and applying a time delay to the secondtransmission signal to generate a delayed second transmission signal;performing time division multiplexing on the first transmission signaland the delayed second transmission signal and generating 2-channelmultiplexed signal; modulating the multiplexed signal by a modulationsignal having a frequency (F−Δf) Hz, when Δf is defined as a real numbersatisfying the condition Δf>0 and F is defined as a real numbersatisfying the condition F>Δf, and generating a modulated multiplexedsignal; converting the modulated multiplexed signal into a modulatedmultiplexed electrical signal and outputting the modulated multiplexedelectrical signal; changing the time delay amount applied to the secondtransmission signal, and determining the time delay amount Δt which thestrength of a Δf Hz frequency component of the modulated multiplexedelectrical signal is minimized; and sequentially adding the time delayamounts (1/2^(N-1))Δt, (2/2^(N-1))Δt, (3/2^(N-1))Δt, . . . , and{(2^(N)−1)/2^(N-1)}Δt which are integral multiples of the time delayamount Δt/2^(N-1) determined on the basis of the determined time delayamount Δt, to the transmission signals of the second to 2^(N)-thchannels, generating delayed second to 2^(N)-th transmission signals,and adjusting the transmission signals of the individual channels suchthat they are arranged at equidistant intervals on a time axis; andperforming time division multiplexing on the transmission signal of thefirst channel and the delayed second to 2^(N)-th transmission signals towhich the time delay amounts are respectively added, and generating amultiplexed transmission signal.

The OTDM/OCDM transmission signal generating method that generates the2^(N)-channel OTDM/OCDM transmission signals is realized by an OTDM/OCDMtransmission signal generating apparatus described below.

The OTDM/OCDM transmission signal generating apparatus according toanother aspect of the invention is an apparatus for generatingtime-division multiplexed encoded transmission signals, including: firstto 2^(N)-th (N is an integer of 1 or more) encoded transmission signalgenerators that encode optical pulse signals whose bit rate is F bit/s,and output generated transmission signals; optical delayers that aredisposed in the second to 2^(N)-th encoded transmission signalgenerators, respectively, to apply time delays to the transmissionsignals output from the second to 2^(N)-th encoded transmission signalgenerators; an optical multiplexer that performs time divisionmultiplexing on the transmission signals output from the first to2^(N)-th encoded transmission signal generators and generates amultiplexed transmission signal; a sub-optical multiplexer that performstime division multiplexing on first and second transmission signalsoutput from first and second encoded transmission signal generatorsselected from the first to 2^(N)-th encoded transmission signalgenerators and generates a 2-channel multiplexed signal; an opticalmodulator that modulates the multiplexed signal by a modulation signalwhose frequency is (F−Δf) Hz, when Δf is defined as a real numbersatisfying the condition Δf>0 and F is defined as a real numbersatisfying the condition F>Δf, and generates a modulated multiplexedsignal; a spectrum analyzer that detects a strength of a Δf Hz frequencycomponent of a multiplexed signal, which changes according to a changein the time delay amount applied to the second encoded transmissionsignal; and an optical delay amount controller that changes the timedelay amount applied to the second encoded transmission signal,determines a time delay amount Δt which the strength of a Δf Hzfrequency component of a modulated multiplexed electrical signal isminimized, sequentially sets the time delay amounts (1/2^(N-1))Δt,(2/2^(N-1))Δt, (3/2^(N-1))Δt, . . . , and {(2^(N)−1)/2^(N-1)}Δt whichare integral multiples of the time delay amount Δt/2^(N-1) determined onthe basis of the determined time delay amount Δt, to the opticaldelayers of the second to 2^(N)-th encoded transmission signalgenerators, and adjusts the transmission signals output from the firstto 2^(N)-th encoded transmission signal generators such that they arearranged at equidistant intervals on a time axis.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a block diagram illustrating the schematic configuration of a2-channel OTDM/OCDM transmission signal generating apparatus accordingto an exemplary embodiment of the invention;

FIG. 2 is a block diagram illustrating the schematic configuration of a2^(N)-channel OTDM/OCDM transmission signal generating apparatusaccording to an exemplary embodiment of the invention;

FIG. 3 is a diagram illustrating the schematic configuration of anoptical multiplexer that is included in the 2^(N)-channel OTDM/OCDMtransmission signal generating apparatus according to the exemplaryembodiment of the invention;

FIG. 4 illustrates time waveforms of signals that are output from anoptical pulse light source, an optical modulator of a first channel, afirst encoded transmission signal generator, and a second encodedtransmission signal generator, respectively;

FIG. 5 illustrates a change between time waveforms of 2-OTDM/OCDMsignals, when positions of time slots allocated to first and secondchannels on a time axis are arranged at an equivalent interval and anon-equivalent interval, respectively;

FIG. 6 is a diagram illustrating a relationship between the time delayamount Δt and the strengths of signal components that are included in anelectric modulation OTDM/OCDM signal and have a frequency of Δf Hz;

FIG. 7A is a diagram illustrating time waveforms of 2-OTDM/OCDM signalswhen the time delay amount Δt is set as 21.4 ps;

FIG. 7B is a diagram illustrating time waveforms of 2-OTDM/OCDM signalswhen the time delay amount Δt is set as 47.9 ps;

FIG. 7C is a diagram illustrating time waveforms of 2-OTDM/OCDM signalswhen the time delay amount Δt is set as 71.2 ps; and

FIG. 7D is a diagram illustrating time waveforms of 2-OTDM/OCDM signalswhen the time delay amount Δt is set as 94.9 ps.

DETAILED DESCRIPTION

As a most basic form of time-division multiplexing communication, thecase where transmission signals of two channels are time-divisionmultiplexed and transmitted is assumed. That is, the case to beconsidered is the one in which an optical pulse signal whose bit rate isF bit/s is encoded and converted into an encoded transmission signal andtransmission signals of two channels are time-division multiplexed andtransmitted. In this case, if encoded transmission signals of first andsecond channels are arranged at an equivalent interval on a time axis, a2F Hz frequency component of a generated time-division multiplexedencoded signal (hereinafter, simply referred to as OTDM/OCDM signal) ismaximized.

When the encoded transmission signals of the first and second channelsare time-division multiplexed, a time delay Δt is applied to the encodedtransmission signal of the second channel before multiplexing. A 2F Hzfrequency component of an OTDM/OCDM signal is maximized by adjusting thetime delay amount Δt. As a result, a state where time slots are equallyallocated to the first and second channels is obtained.

That is, the time delay amount Δt in which the 2F Hz frequency componentof the OTDM/OCDM signal is maximized is added to the encodedtransmission signal of the second channel and the encoded transmissionsignal is time-division multiplexed. The time slots may be equallyallocated to the first and second channels, and the time slots allocatedto the individual channels may be set to the minimum widths.

Even when the number of time-division multiplexed channels is 2^(N) (Nis an integer of 1 or more), encoded transmission signals of 2 channelsthat are selected from the 2^(N) channels are multiplexed and 2-channelOTDM/OCDM signals (hereinafter, simply referred to as 2-OTDM/OCDMsignals) are generated. As described above, the time delay amount ΔTwhere the time slots are equally allocated to the 2 selected channels iscalculated. Thereby, time delays of (1/2^(N-1))Δt, (2/2^(N-1))Δt,(3/2^(N-1))Δt, {(2^(N)−1)/2^(N-1)}Δt, which are equal to the integralmultiple of the time delay amount Δt/2^(N-1), are sequentially appliedto the encoded transmission signals of the second and followingchannels, and the encoded transmission signal of the first channelbecoming a reference is time-division multiplexed. As a result, theencoded transmission signals of the first to 2^(N)-th channels arearranged at an equivalent interval on a time axis.

However, it is very difficult to observe the component of the OTDM/OCDMsignal where the frequency is 2F Hz, because a value of F is a largevalue reaching up to the range of GHz.

Therefore, the inventors of the invention have modulated 2-OTDM/OCDMsignals by modulation signals having frequency (F−Δf) Hz, and havemeasured the strength of Δf Hz frequency component of the generatedmodulated OTDM/OCDM signals. As a result, the inventors noticed that the2F Hz frequency component of the OTDM/OCDM signal may be indirectlymeasured. In this case, Δf is defined as a real number satisfying thecondition Δf>0 and F is defined as a real number satisfying thecondition F>Δf.

An optical pulse signal whose bit rate is F bit/s is encoded to generatean encoded signal, and a component where a frequency is F Hz is includedin an OTDM/OCDM signal obtained by performing time division multiplexingon the encoded signal. The OTDM/OCDM signal is modulated by a modulationsignal having frequency (F−Δf) Hz, and a Δf Hz frequency component isincluded in the generated modulated OTDM/OCDM signal.

The strength of the Δf Hz frequency component and the strength of the 2FHz frequency component of the OTDM/OCDM signal are related in such a waythat, if the strength of one of the two components increases, thestrength of the other decreases. That is, in order to adjust theabove-described time delay amount Δt, the strength of the component ofthe Δf Hz frequency component of the modulated OTDM/OCDM signal may beobserved, instead of observing the strength of the 2F Hz frequencycomponent of the OTDM/OCDM signal.

According to an OTDM/OCDM transmission signal generating methodaccording to an aspect of the invention, a 2-OTDM/OCDM signal ismodulated by a modulation signal having a frequency (F−Δf) Hz, and thestrength of a Δf Hz frequency signal component that is included in the2-OTDM/OCDM signal modulated by the modulation signal is detected.

In this case, in a frequency spectrum of the 2-OTDM/OCDM signal, an F Hzfrequency component is a main component. This is represented as A sin2πFt, if the magnitude of amplitude is defined as A and a time variableis defined as t. A modulation signal having a frequency (F−Δf) Hz isrepresented as B sin 2π(F−Δf)t, if the amplitude of the modulationsignal is defined as B.

If the 2-OTDM/OCDM signal is modulated by the modulation signal havingfrequency (F−Δf) Hz, as represented by the following Equation (1), asignal that is given by a sum of a signal component where a frequency is(2F−Δt) Hz and a signal component where a frequency is Δf Hz isobtained.A sin 2πFt×B sin 2π(F−Δf)t=(AB/2)cos 2πΔft−(AB/2)cos 2π(2F−Δf)t  (1)

When the time delay amount is determined, the time delay amount of thesecond encoded transmission signal (encoded transmission signal of thesecond channel) with respect to the first encoded transmission signal(encoded transmission signal of the first channel) is changed, thestrength of a Δf Hz signal component that is included in the 2-OTDM/OCDMsignal modulated by the modulation signal having the frequency (F−Δf) Hzis detected, and the time delay amount Δt where the strength of the ΔfHz signal component is minimized is determined.

Since the frequencies of the first and second encoded transmissionsignals are F Hz, if the first and second encoded transmission signalsare arranged at an equivalent interval on the time axis, the 2F Hzfrequency component included in the 2-OTDM/OCDM signal is maximized, andthe F Hz frequency component is minimized.

When the F Hz frequency component is minimized, in Equation (1), a valueof A that is an amplitude coefficient of A sin 2πFt is minimized. Inthis case, the magnitude of an amplitude coefficient (AB/2) of (AB/2)cos2πΔft that is a first term of a right side of Equation (1) is alsominimized, when the value of A is minimized.

Accordingly, the first encoded transmission signal and the secondencoded transmission signal are arranged at an equivalent interval onthe time axis. In the case where the time delay amount Δt is added tothe second encoded transmission signal such that a 2F Hz frequencycomponent of the 2-OTDM/OCDM signal is maximized, if the amplitudecoefficient (AB/2) of the first term of the right side that is given by(AB/2)cos 2πΔft of Equation (1) has a minimum value, the signal strengthof the Δf Hz frequency component is minimized.

To determine the time delay amount, the time delay amount of the secondencoded transmission signal with respect to the first encodedtransmission signal is changed so as to determine the time delay amountin which the strength of the Δf Hz signal component is minimized. In amethod that generates OTDM/OCDM transmission signals of 2^(N) channels,on the basis of the determined time delay amount Δt, time delay amounts(1/2^(N-1))Δt, (2/2^(N-1))Δt, (3/2^(N-1))Δt, {(2^(N)−1)/2^(N-1)}Δt thatare obtained by multiplying the time delay amount Δt/2^(N-1) by integersare sequentially applied to the second to 2^(N)-th encoded transmissionsignals, and delayed second to 2^(N)-th encoded transmission signals aregenerated.

As described above, according to the OTDM/OCDM transmission signalgenerating method, 2-OTDM/OCDM signals that are generated bymultiplexing encoded transmission signals of 2 channels that areselected from encoded transmission signals of the number of multiplexedchannels are modulated by modulation signals having a frequency (F−Δf)Hz, and the time delay amount Δt in which the strength of Δf Hzfrequency component of the modulated OTDM/OCDM signals is minimized, isdetermined. On the basis of the determined time delay amount Δt, timedelay is applied to an encoded transmission signal of each of themultiplexed channels and the encoded transmission signal istime-division multiplexed. As a result, the encoded transmission signalsof the individual channels are arranged at an equivalent interval on thetime axis.

If the ΔHz component of the modulated OTDM/OCDM signal with respect tothe time delay amount Δt applied to the second encoded transmissionsignal is monitored, the time delay is applied to an encodedtransmission signal of each of the multiplexed channels on the basis ofthe time delay amount Δt applying the minimum value of the Δf Hzcomponent, and the encoded transmission signal is time-divisionmultiplexed, the encoded transmission signals of the individual channelsare always arranged at an equivalent interval on the time axis.

In the case of an OTDM/OCDM transmission signal generating apparatusthat generates OTDM/OCDM transmission signals of 2^(N) channels, iffirst and second encoded transmission signals are allocated as encodedtransmission signals of 2 channels that are selected from encodedtransmission signals of the number of multiplexed channels, the firstand second encoded transmission signals are always arranged at anequivalent interval on the time axis.

Hereinafter, an exemplary embodiment of the invention will be describedwith reference to the accompanying drawings. The individual drawingsthat are provided to describe a form of the OTDM/OCDM transmissionsignal generating apparatus and a form of portions constituting theOTDM/OCDM transmission signal generating apparatus illustrate oneconfiguration according to the invention. Accordingly, the arrangementof the individual constituents is schematically illustrated to a degreeto which the invention may be understood, and the invention is notlimited to the examples illustrated in the drawings. In the individualdrawings, the same constituents are denoted by the same referencenumerals, and the redundancy is omitted. In the schematic blockconfiguration drawings illustrated below, a path of an optical signal,such as an optical fiber, is illustrated by a thick line, and a path ofan electric signal is illustrated by a thin line.

<OTDM/OCDM Transmission Signal Generating Apparatus>

In order to describe the basic configuration of the OTDM/OCDMtransmission signal generating apparatus according to the invention,first, an exemplary embodiment of the OTDM/OCDM transmission signalgenerating apparatus that generates OTDM/OCDM transmission signals of 2channels will be described.

The configuration and operation of the 2-channel OTDM/OCDM transmissionsignal generating apparatus according to the exemplary embodiment of theinvention will be described with reference to FIG. 1. FIG. 1 is a blockdiagram illustrating the schematic configuration of the 2-channelOTDM/OCDM transmission signal generating apparatus according to theexemplary embodiment of the invention.

The 2-channel OTDM/OCDM transmission signal generating apparatusaccording to the exemplary embodiment of the invention includes a firstencoded transmission signal generator 20, a second encoded transmissionsignal generator 30, an optical delayer 40, an optical multiplexer 42,an optical branching device 44, an optical modulator 46, a photoelectricconverter 48, a spectrum analyzer 50, and an optical delay amountcontroller 52. Hereinafter, the configuration and operation of theOTDM/OCDM transmission signal generating apparatus according to theexemplary embodiment of the invention will be described in detail,including the constituents and other elements needed to operate theconstituents.

A basic clock signal 61 having F Hz, which is a basic operationfrequency, is output from an oscillator 60 to the 2-channel OTDM/OCDMtransmission signal generating apparatus. The basic clock signal 61 isbranched into basic clock signals 63-1 and 63-2 by an electric branchingdevice 62. The basic clock signal 63-1 is supplied to a pulse lightsource 10, and an optical pulse train 11 where a frequency is F Hz isrepetitively output from the pulse light source 10.

Meanwhile, the basic clock signal 63-2 is supplied to a mixer 66 andmixed with a sine-wave signal 65 that is output from the oscillator 64and has a frequency of Δf Hz, and a source modulation signal 67 that isgiven by the above Equation (1) is generated. Only a (F−Δf) Hz frequencycomponent of the source modulation signal 67 is filtered by a band-passfilter 68, and the source modulation signal 67 is used as a modulationsignal of the optical modulator 46 to be described in detail below.

The optical pulse train 11 that is output from the pulse light source 10is input to an optical branching filter 12, and is branched into a firstoptical pulse train 13-1 and a second optical pulse train 13-2 andoutput. The first optical pulse train 13-1 is input to the first encodedtransmission signal generator 20.

The first encoded transmission signal generator 20 includes a datasignal generator 14, an optical modulator 16, and an encoder 18. Forconvenience of explanation, it is assumed that the first channel isallocated to the first encoded transmission signal generator 20 and thesecond channel is allocated to the second encoded transmission signalgenerator 30 to be described in detail below.

In the description below, the first encoded transmission signal and thesecond encoded transmission signal may both be simply called encodedtransmission signals provided that it does not cause confusion.

The data signal generator 14 outputs a binary digital electric signal 15that is a transmission data signal of the first channel. The opticalmodulator 16 is modulated by the electric signal 15, and the firstoptical pulse train 13-1 is converted into a first optical pulse signal17 and is output from the optical modulator 16. The first optical pulsesignal 17 is encoded by a code that is allocated to the first channel bythe encoder 18, and the first encoded transmission signal 19 isgenerated and output. That is, the first encoded transmission signalgenerator 20 encodes the optical pulse signal 17 of the first channelwhose bit rate is F bit/s, and generates the first encoded transmissionsignal 19. In FIG. 1, since the first channel is allocated, the firstencoded transmission signal generator 20 is displayed as CH-1

The second encoded transmission signal generator 30 includes a datasignal generator 34, an optical modulator 34, an encoder 38, and anoptical delayer 40.

The data signal generator 34 outputs the binary digital electric signal35 that is the transmission data signal of the second channel. Theoptical modulator 36 is modulated by the electric signal 35, and thesecond optical pulse train 13-2 is converted into the second opticalpulse signal 37 and output from the optical modulator 36. The secondoptical pulse signal 37 is encoded by a code that is allocated to thesecond channel by the encoder 38, and the second encoded transmissionsignal 39 is generated and output. The second encoded transmissionsignal 39 is input to the optical delayer 40, adds the time delay amountΔt needed to arrange the encoded transmission signals of the first andsecond channels at an equivalent interval on the time axis, andgenerates the second encoded transmission signal 41.

The second encoded transmission signal generator 30 encodes the opticalpulse signal 37 of the second channel whose bit rate is F bit/s, andgenerates the second encoded transmission signal 39. The second encodedtransmission signal 41 is output from the optical delayer 40 that isincluded in the second encoded transmission signal generator 30. Thatis, the second encoded transmission signal 41 is output from the secondencoded transmission signal generator 30. In FIG. 1, since the secondchannel is allocated, the second encoded transmission signal generator30 is displayed as CH-2.

The first encoded transmission signal 19 and the second encodedtransmission signal 41 are input to the optical multiplexer 42 and a2-OTDM/OCDM signal 43 is output. The 2-OTDM/OCDM signal 43 is input tothe optical branching device 44, branched into a 2-OTDM/OCDM signal 45-1and an OTDM/OCDM signal 45-2 for monitoring, and output. The 2-OTDM/OCDMsignal 45-1 is used as a signal that is transmitted as the 2-OTDM/OCDMtransmission signal.

Meanwhile, the OTDM/OCDM signal 45-2 for monitoring is input to theoptical modulator 46 and modulated by a modulation signal 71 having afrequency (F−Δf) Hz, and a modulated OTDM/OCDM signal 47 is generatedand output. As the modulation signal that modulates the opticalmodulator 46, a modulation signal 69 that is output from a band-passfilter 68 and has a frequency of (F−Δf) Hz may be used. However, thestrength of the modulation signal 69 may not be sufficient according tothe used optical modulator 46. Accordingly, the modulation signal 71that is amplified by the amplifier 70 and obtained may be used as themodulation signal.

The optical branching device 44 may be an optical branching device of atype that taps a portion of the 2-OTDM/OCDM signal 43 and extracts theOTDM/OCDM signal 45-2 for monitoring or may be a type of an opticalswitching element that switches the 2-OTDM/OCDM signal 43 into an2-OTDM/OCDM signal 45-1 and an OTDM/OCDM signal 45-2 for monitoring andoutputs the signals.

When the optical branching device of the type that taps the portion ofthe signal and extracts the signal is used, the 2-OTDM/OCDM signal 45-1is always output from the OTDM/OCDM transmission signal generatingapparatus according to the exemplary embodiment of the invention. Whenthe type of an optical switching element is used, during the time inwhich a Δf signal component is extracted, a time delay amount isdetermined, and an optical delay amount is set, the 2-OTDM/OCDM signal45-1 is not output. The optical branching device of the type tapping theportion of the signal and extracting the signal or the type of anoptical switching element is used according to design criteria.

The modulated OTDM/OCDM signal 47 is input to the photoelectricconverter 48, converted into the electrical modulation OTDM/OCDM signal49, and output. The electrical modulation OTDM/OCDM signal 49 is inputto the spectrum analyzer 50, and the strength of the Δf Hz signalfrequency component in the electrical modulation OTDM/OCDM signal 49 isdetected. The strength of the Δf Hz signal component is output from thespectrum analyzer 50. Accordingly, the delay time Δt for which thestrength of the Δf Hz signal component is minimized may be found as thedelay time Δt provided to the optical delayer 40 is continuouslychanged.

If the optical delay amount controller 52 instructs the optical delayer40 using an instruction signal 53 to provide the delay time Δt to theoptical delayer 40, the delay time Δt that enables the encodedtransmission signals of the first and second channels to be arranged atan equivalent interval on the time axis is added to the second encodedtransmission signal 39.

The Δf signal component extracting step, the time delay amountdetermining step, and the optical delay amount setting step that arerespectively executed mainly using the optical modulator 46, thespectrum analyzer 50, and the optical delay amount controller 52 maybemanually executed. If the strength of the Δf Hz signal component isvisually observed by the spectrum analyzer 50 while the delay time Δtprovided to the optical delayer 40 is continuously changed, in order tofind the delay time Δt for which the strength of the Δf Hz signalcomponent is minimized. If the delay time Δt is manually provided to theoptical modulator 40, the object is achieved.

The Δf signal component extracting step, the time delay amountdetermining step, and the optical delay amount setting step may beautomated. In FIG. 1, a designation signal 51 that designates the delaytime Δt for which the strength of the Δf Hz signal component isminimized is output from the spectrum analyzer 50, and the optical delayamount controller 52 instructs the optical delayer 40 using theinstruction signal 53 to provide the delay time Δt to the opticaldelayer 40 on the basis of the designation signal 51. The automation maybe appropriately executed by those skilled in the art using a method,such as known programming, on the basis of the above description.

Next, an exemplary embodiment of the OTDM/OCDM transmission signalgenerating apparatus that generates the 2^(N)-channel OTDM/OCDMtransmission signals will be described with reference to FIGS. 2 and 3.FIG. 2 is a block diagram illustrating the schematic configuration of a2^(N)-channel OTDM/OCDM transmission signal generating apparatusaccording to an exemplary embodiment of the invention.

The 2^(N)-channel OTDM/OCDM transmission signal generating apparatusillustrated in FIG. 2 is different from the 2-channel OTDM/OCDMtransmission signal generating apparatus illustrated in FIG. 1 in thatthe 2^(N)-channel OTDM/OCDM transmission signal generating apparatusdoes not include only the two encoded transmission signal generators,but includes a first encoded transmission signal generator 120-1, asecond encoded transmission signal generator 120-2, . . . , and a2^(N)-th encoded transmission signal generator 120-2 ^(N), whichcorrespond to the number allocated to 2^(N) channels. The first encodedtransmission signal generator 120-1 has the same structure as the firstencoded transmission signal generator 20 illustrated in FIG. 1. Thesecond to 2^(N)-th encoded transmission signal generators 120-2 to 120-2^(N) have the same structure as the second encoded transmission signalgenerator 30 illustrated in FIG. 1.

The second to 2^(N)-th encoded transmission signal generators includeoptical delayers 140-2 to 140-2 ^(N), respectively. However, theinvention is not limited to this configuration, and the optical delayers140-2 to 140-2 ^(N) may be integrated as the optical delayer 140.

The optical multiplexer 142 performs time division multiplexing on thefirst encoded transmission signal 121-1 output from the first encodedtransmission signal generator 120-1 and the delayed second encodedtransmission signal 121-2 to the delayed 2^(N)-th encoded transmissionsignal 121-2 ^(N) output from the second to 2^(N)-th encodedtransmission signal generators, and outputs the OTDM/OCDM transmissionsignal 143. In the description below, the delayed second encodedtransmission signal 121-2 to the delayed 2^(N)-th encoded transmissionsignal 121-2 ^(N) may be simplified and may be called an encodedtransmission signal provided that it does not cause confusion.

In this case, in the first encoded transmission signal 121-1 or thedelayed 2^(N)-th encoded transmission signal 121-2 ^(N), “-” that isdisplayed in 121-1 or 121-2 ^(N) to identify the encoded transmissionsignal is not a subtraction symbol that means a subtraction from 121,but it simply denotes a hyphen. Meanwhile, “-” between “N” and “1” inΔt/2^(N-1) is a subtraction symbol, which means that 1 is subtractedfrom a numerical value N. Since whether the symbol is the hyphen or thesubtraction symbol is apparent from an anteroposterior relationship of asentence or Equation, the specific description is omitted.

The optical branching filter 112 and the optical multiplexer 142illustrated in FIG. 2 that correspond to the optical branching filter 12and the optical multiplexer 42 illustrated in FIG. 1 are configuredaccording to the number of optical signals to be branched andmultiplexed, to correspond to the increase in the number oftime-division multiplexed channels. Therefore, the basic configurationis the same. Since the constituents that are denoted by the samereference numerals are the same as those of the 2-channel OTDM/OCDMtransmission signal generating apparatus illustrated in FIG. 1, theredundancy in the description of the same constituents are omitted.

Even in this case, the Δf signal component extracting step, the timedelay amount determining step, and the optical delay amount setting stepare executed mainly using the optical modulator 46, the spectrumanalyzer 50, and the optical delay amount controller 152.

The configuration of the optical multiplexer 142 will be described withreference to FIG. 3. FIG. 3 is a diagram illustrating the schematicconfiguration of the optical multiplexer 142 that is included in the2^(N)-channel OTDM/OCDM transmission signal generating apparatusaccording to the exemplary embodiment of the invention.

The optical multiplexer 142 receives a first encoded transmission signal121-1 that is output from the first encoded transmission signalgenerator 120-1 and a delayed second encoded transmission signal 121-2to a delayed 2^(N)-the encoded transmission signal 121-2 ^(N) that areoutput from the second to 2^(N)-th encoded transmission signalgenerators 120-2 to 120-2 ^(N).

In this case, it is assumed that the first encoded transmission signalgenerator 120-1 and the second encoded transmission signal generator120-2 are selected as the first and second encoded transmission signalgenerators selected from the first to 2^(N)-th encoded transmissionsignal generators.

The optical multiplexer 142 includes N optical couplers 142-1 to 142-Nof two inputs and two outputs. Each of the optical couplers 142-1 to142-N is coupled to the two encoded transmission signal generators amongthe sequentially adjacent encoded transmission signal generators. Forexample, the optical coupler 142-1 multiplexes the first encodedtransmission signal 121-1 that is output from the first encodedtransmission signal generator 120-1 and the second encoded transmissionsignal 121-2 that is output from the second encoded transmission signalgenerator 120-2 to generate 2 multiplexed OTDM/OCDM signals, and dividesthe 2 multiplexed OTDM/OCDM signals into two parts with the equivalentstrength and outputs the divided signals.

Similar to the optical coupler 142-1, each of the optical couplers 142-2to 142-N multiplexes the delayed encoded transmission signals that areoutput from the sequentially adjacent encoded transmission signalgenerators to generate 2 multiplexed OTDM/OCDM signals, and divides the2 multiplexed OTDM/OCDM signals into two parts with the equivalentstrength and outputs the divided signals.

Even in the rear stages of the optical couplers 142-2 to 142-N, similarto the above case, 2^(N) time-division multiplexed encoded signals(2^(N)-OTDM/OCDM signals) 143-2 where the first encoded transmissionsignal 121-1 to the delayed 2^(N)-th encoded transmission signal 121-2^(N) are time-division multiplexed are generated by disposing theoptical couplers of two inputs and two outputs in a cascade manner. Thatis, the first encoded transmission signal 121-1 and the delayed secondto 2^(N)-th encoded transmission signals are input to the opticalmultiplexer 142, and the OTDM/OCDM transmission signals 143 are output.

The optical coupler 142-1 multiplexes the first encoded transmissionsignal 121-1 that is output from the first encoded transmission signalgenerator and the delayed second encoded transmission signal 121-2 thatis output from the second encoded transmission signal generator 120-2 togenerate 2 multiplexed OTDM/OCDM signals, and one 2-OTDM/OCDM signal143-1 that divides the 2-multiplexed OTDM/OCDM signals into two partshaving the equivalent strength and outputs the divided signals is outputfrom the optical multiplexer 142. In FIG. 2, the 2-OTDM/OCDM signal143-1 and the 2^(N)-OTDM/OCDM signal 143-2 are collected and simplifiedas the OTDM/OCDM signal 143. The 2-OTDM/OCDM signal 143-1 and the2^(N)-OTDM/OCDM signal 143-2 are input to the optical branching device144, branched into a 2^(N)-OTDM/OCDM signal 145-1 and an OTDM/OCDMsignal 145-2 for a monitor, and output. The 2^(N)-OTDM/OCDM signal 145-1is used as the signal that is transmitted as the 2^(N)-OTDM/OCDM signal145-1.

In FIG. 3, the optical coupler 142-1 performs a function of asub-optical multiplexer that multiplexes the first and second encodedtransmission signals output from the first and second encodedtransmission signal generators selected from the first to 2^(N)-thencoded transmission signal generators 120-1 to 120-2 ^(N) and generates2-OTDM/OCDM signals.

The optical modulator 46 modulates the OTDM/OCDM signal 145-2 formonitoring by a modulation signal 71 having a frequency (F−Δf) Hz andoutputs a modulated OTDM/OCDM signal 147.

The spectrum analyzer 50 detects the strength of the Δf Hz signalcomponent in the modulated OTDM/OCDM signal 147, which changes accordingto the change in the time delay amount that is added to the secondencoded transmission signal.

The second encoded transmission signal generator 120-2 has the sameconfiguration as the second encoded transmission signal generator 30that is described with reference to FIG. 1. The second encodedtransmission signal generator 120-2 includes an optical delayer 140-2.The optical delayer 140-2 adds the time delay to the second encodedtransmission signal that corresponds to the second encoded transmissionsignal 39 in the second encoded transmission signal generator 30illustrated in FIG. 1, and the delayed second encoded transmissionsignal is generated.

The optical delay amount controller 152 adjusts the encoded transmissionsignals 121-1 to 121-2 ^(N), which are output from the first encodedtransmission signal generator 120-1 to the 2^(N)-th encoded transmissionsignal generator 120-2 ^(N), such that they are arranged at anequivalent interval on the time axis. For this reason, the time delayamounts (1/2^(N-1))Δt, (2/2^(N-1))Δt, (3/2^(N-1))Δt,{(2^(N)−1)/2^(N-1)}Δt equal to the integral multiple of the time delayamount Δt/2^(N-1), which are determined on the basis of the time delayamount Δt applied to the second encoded transmission signal where thestrength of the Δf Hz signal component is minimized, are controlled tobe sequentially set to the optical delayers of the second to 2^(N)-thencoded transmission signal generators 120-2 to 120-2 ^(N).

<Operation Principle for Determining the Time Delay Amount Δt Added tothe Encoded Transmission Signal>

The operation principle for determining the time delay amount Δt addedto the encoded transmission signal will be described below withreference to FIGS. 1 and 4 to 8.

As illustrated in FIG. 1, the optical pulse train 11, the first opticalpulse signal 17, the first encoded transmission signal 19, and the2-OTDM/OCDM signal 43 are output from the optical pulse light source 10,the optical modulator 16 of the first channel, the first encodedtransmission signal generator 20, and the optical multiplexer,respectively.

FIG. 4 illustrates time waveforms of signals that are output from theoptical pulse light source 10, the optical modulator 16 of the firstchannel, the first encoded transmission signal generator 20, and theoptical multiplexer, respectively. (1) schematically illustrates a timewaveform of the optical pulse train 11, (2) schematically illustrates atime waveform of the first optical pulse signal 17, (3) schematicallyillustrates a time waveform of the first encoded transmission signal 19,and (4) schematically illustrates a time waveform of the 2-OTDM/OCDMsignal 43. In FIG. 4, a horizontal axis represents a time axis with anarbitrary scale using scale marks and a vertical axis represents lightstrength with an arbitrary scale using scale marks.

In FIG. 1, parenthesized numbers (1) to (4) are added to the adjacentright sides of reference numerals (11, 17, 19, and 43) indicating theoptical pulse train 11, the first optical pulse signal 17, the firstencoded transmission signal 19, and the 2-OTDM/OCDM signal 43 tocorrespond to the time waveforms of FIG. 4.

The optical pulse train 11 is two-branched into a first optical pulsetrain 13-1 and a second optical pulse train 13-2 by the opticalbranching filter 12, and the first optical pulse train 13-1 and thesecond optical pulse train 13-2 are supplied to the first encodedtransmission signal generator 20 and the second encoded transmissionsignal generator 30. The strengths of the first optical pulse train 13-1and the second optical pulse train 13-2 are different from the strengthof the optical pulse train 11, but the time waveforms of the firstoptical pulse train 13-1 and the second optical pulse train 13-2 arealmost the same as the waveform of the optical pulse train 11.Accordingly, in the description below, the time waveforms of the firstoptical pulse train 13-1 and the second optical pulse train 13-2 may bedescribed as the time waveform of the optical pulse train 11, providedthat it does not cause confusion.

As illustrated in FIG. 4, (1), the optical pulse train 11 is an opticalpulse train that is output from the pulse light source 10 and has arepetitive frequency of F Hz. Accordingly, the time interval of theadjacent optical pulses is 1/F sec. For example, when the optical pulsetrain 11 is an optical pulse train that is output from the pulse lightsource 10 and has a repetitive frequency of F GHz, the time interval ofthe adjacent optical pulses becomes 1/F ps (picosecond).

FIG. 4, (2) illustrates the first optical pulse signal 17 on theassumption that the first channel is allocated to the first encodedtransmission signal generator 20 and a binary digital electric signal 15corresponding to a transmission data signal of the first channel is (0,1, 0, 0, 1, 0, 1, 1, 1, . . . ). In the optical modulator 16, if theoptical pulse train 11 is modulated by the electric signal 15, the firstoptical pulse signal 17 that has the time waveform where the electricsignal 15 is reflected is output from the optical modulator 16.

FIG. 4, (2) illustrates a rectangular-wave signal indicating whether theoptical modulator 16 is in a transmissive state or a blocked statesuperposed on the optical pulse train 11. The rectangular-wave signalhas the time waveform of the binary digital electric signal 15. The timewidth of one rectangular wave that indicates the time waveform becomessmaller than or equal to the time slot that is distributed for eachchannel.

As illustrated in FIG. 4, (3), the first encoded transmission signal 19is generated by encoding performed when one optical pulse temporallyspreads to plural chip pulses in the time slot allocated to the firstchannel. FIG. 4, (3) illustrates plural minute optical pulses (chippulses) with respect to one optical pulse. However, in a chip pulsetrain, the code that is set to the encoder 18 included in the firstencoded transmission signal generator 20 is reflected. FIG. 4, (3)schematically illustrates a state where the detailed structure of thecode is omitted, and one optical pulse is encoded as a group of theplural chip pulses and converted.

FIG. 4, (4) illustrates a time waveform of the 2-OTDM/OCDM signal 43where the first encoded transmission signal 19 and the second encodedtransmission signal 41 are multiplexed by the optical multiplexer 42 andoutput. The second encoded transmission signal 41 is generated by addingthe time delay to the second encoded transmission signal 39 by theoptical delayer 40. The second encoded transmission signal 39 isgenerated by encoding the second optical pulse signal 37, which isgenerated by modulating the second encoded transmission signal 39 by thebinary digital electric signal 35 corresponding to the transmission datasignal of the second channel, by the encoder 38 included in the secondencoded transmission signal generator 30.

In this case, the process in which the second optical pulse train 13-2of the second channel is modulated by the electric signal 35 and thesecond optical pulse signal 37 is generated, and the second opticalpulse signal 37 is encoded by the encoder 38 and the second encodedtransmission signal 39 is generated is the same as the process in whichthe first encoded transmission signal 19 of the first channel isgenerated. Accordingly, the provisions of the time waveforms in theprocesses are omitted. In this case, on the assumption that the electricsignal 35 is (1, 1, 0, 0, 1, 0, 1, 0, 1, . . . ), the component of thesecond encoded transmission signal 41 is illustrated in FIG. 4, (1). InFIG. 4, (4), the component of the first encoded transmission signal 19is denoted by “CH-1” and the component of the second encodedtransmission signal 41 is denoted by “CH-2”, such that the component ofthe first encoded transmission signal 19 and the component of the secondencoded transmission signal 41 can be easily distinguished from eachother. The time waveform that is illustrated in FIG. 4, (3) is the timewaveform of the first encoded transmission signal 19. Accordingly, thetime waveform that is illustrated in FIG. 4, (3) and the time waveformof the component of the first encoded transmission signal 19 illustratedin FIG. 4, (4) overlap each other on the time axis.

Referring to FIG. 5, a situation where the time slots are equivalentlyallocated to the first and second channels by changing the time delayamount Δt set to the optical delayer will be described.

FIG. 5 illustrates a change between a time waveform of a 2-OTDM/OCDMsignal 43, when positions of the encoded transmission signals of thefirst and second channels on a time axis are arranged at an equivalentinterval and a non-equivalent interval, respectively. FIG. 5, (1)illustrates a time waveform of the 2-OTDM/OCDM signal 43 when thepositions of the time slots on the time axis overlap at the sameposition in the first and second channels. FIG. 5, (2) illustrates atime waveform of the 2-OTDM/OCDM signal 43 when the positions of theencoded multiplex signals of the first and second channels on the timeaxis are set at an equivalent interval by adding the time delay to thesecond encoded transmission signal 39 by the optical delayer 40.

The second encoded transmission signal 41 is generated by adding thetime delay to the second encoded transmission signal 39 by the opticaldelayer 40. Accordingly, depending on a value of the time delay amountΔt that is applied by the optical delayer 40, in the time waveform ofthe 2-OTDM/OCDM signal 43, the positions of the time slots on the timeaxis overlap each other at the same position in the first and secondchannels, as illustrated in FIG. 5, (1). In FIG. 5, (1), “CH-1(CH-2)” isdisplayed to indicate that the time slots of the first and secondchannels overlap each other.

That is, depending on the value of the time delay amount Δt, the chippulse group of the first channel and the chip pulse group of the secondchannel overlap each other on the time axis. Accordingly, if theabove-described time delay amount Δt is adjusted, the positions of thetime slots of the first and second channels on the time axis may be setto be arranged at an equivalent interval. The time waveform of the2-OTDM/OCDM signal 43 when the above state is realized becomes awaveform illustrated in FIG. 5, (2). An object of the invention is tocalculate the value of the time delay amount Δt satisfying the abovecondition. When the positions of the time slots allocated to the firstand second channels on the time axis are arranged at an equivalentinterval, the encoded transmission signals of the first and secondchannels are arranged at an equivalent interval on the time axis.

If FIG. 5, (1) and (2) are compared with each other, the following factmay be observed. That is, as illustrated in FIG. 5, (1), when thepositions of the time slots on the time axis overlap each other at thesame position in the first and second channels, the placement intervalof the chip pulse groups of the first and second channels on the timeaxis is always 1/F Hz. Accordingly, there is no place where theplacement interval is 1/2F Hz.

Meanwhile, as illustrated in FIG. 5, (2), when the placement interval ofthe chip pulse groups of the first and second channels on the time axisis equidistant, the placement interval of the chip pulse groups on thetime axis is mainly 1/2F Hz. In the case where the time waveform becomesthe time waveform illustrated in FIG. 5, (2) by adjusting the time delayamount Δt, the number of places where the placement interval of the chippulse groups on the time axis becomes 1/2F Hz is largest.

Accordingly, when the 2-OTDM/OCDM signal 43 is spectrally decomposed,the frequency components of F Hz and the frequency components of 2F Hzin the 2-OTDM/OCDM signal 43 are changed in response to the time delayamount Δt. For example, if one of the frequency components of F Hz andthe frequency components of 2F Hz increases, the other decreases. Whenthe time waveform becomes the time waveform illustrated in FIG. 5, (2),the strength of the frequency components of 2F Hz is maximized and thestrength of the frequency components of F Hz is minimized.

However, the frequency component of F Hz or the frequency component of2F Hz is an extraordinary high frequency component that is generallyrepresented in a unit of GHz. For this reason, it is difficult toobserve the frequency components using the common spectrum analyzer.Accordingly, the 2-OTDM/OCDM signal 43 is modulated by a modulationsignal having a frequency (F−ΔF) Hz. If the strength of the Δf Hzfrequency component of the generated modulated OTDM/OCDM signal 47 isobserved, the 2F Hz frequency component of the OTDM/OCDM signal isindirectly observed. In this case, Δf is defined as a real numbersatisfying the condition Δf>0 and F is defined as a real numbersatisfying the condition F>Δf.

Accordingly, first, the modulation signal where the frequency is (F−Δf)Hz needs to be generated. A method of generating the modulation signalwill be described with reference to FIG. 1.

The basic clock signal 61 of F Hz is output from the oscillator 60. Thebasic clock signal 61 is branched into basic clock signals 63-1 and 63-2by the electric branching device 62. The basic clock signal 63-2 issupplied to a mixer 66 and mixed with a sine-wave signal 65 that isoutput from the oscillator 64 and has a frequency of Δf Hz, and a sourcemodulation signal 67 is generated.

The basic clock signal 63-2 is a basic clock signal of F Hz.Accordingly, if the magnitude of amplitude is defined as C, the basicclock signal is given by C sin 2πFt. Meanwhile, the sine-wave signal 65where the frequency is Δf Hz is given by D sin 2πΔft, if the magnitudeof amplitude is defined as D. Accordingly, the source modulation signal67 that is output from the mixer 66 is given by the following Equation(2).C sin 2πFt×D sin 2πΔft=(CD/2){cos 2π(F−Δf)t−cos 2π(F+Δf)t}  (2)

The component of the source modulation signal 67 where the frequency is(F−Δf) Hz is selected by the band-pass filter 68, and the modulationsignal 69 having the frequency (F−Δf) Hz is generated and output. Anamplifier 70 is used to amplify the strength of the modulation signal 69to the strength needed to drive the optical modulator. The modulationsignal 71 that is output from the amplifier 70 is a sine-wave signalhaving the frequency (F−Δf) Hz. In this case, the phase of themodulation signal 71 that is the sine-wave signal is ignored, and themagnitude of the amplitude is assumed as B. As a result, the signalwaveform may be represented as B sin 2π(F−Δf)t. In this way, themodulation signal 71 having the frequency (F−Δf) Hz is generated.

The 2-OTDM/OCDM signal 45-2 is modulated using the modulation signal 71having the frequency (F−Δf) Hz by the optical modulator 46. Asrepresented in the above Equation (1), the modulated OTDM/OCDM signal 47that is given, by the sum of the signal component where the frequency is(2F−Δf) Hz and the signal component where the frequency is Δf Hz isobtained. For convenience of explanation, Equation (1) is reproducedbelow.A sin 2πFt×B sin 2π(F−Δf)t=(AB/2)cos 2πΔft−(AB/2)cos 2π(2F−Δf)t  (1)

As described above, if the signal component where the frequency is F Hzis minimized, this corresponds to the case where the value of Acorresponding to the amplitude coefficient of A sin 2ππFt in Equation(1) is minimized. In this case, the magnitude of the amplitudecoefficient (AB/2) of (AB/2)cos 2πΔft that corresponds to the first termof the right side of Equation (1) is minimized, when the value of A isminimized.

Accordingly, the time slots are equivalently distributed to the firstencoded transmission signal 19 and the second encoded transmissionsignal 39, and the time delay amount Δt is added to the second encodedtransmission signal 39 such that the 2F Hz signal frequency component ofthe 2-OTDM/OCDM signal 45-2 is maximized. In this case, if the amplitudecoefficient (AB/2) of the first term of the right side that is given by(AB/2)cos 2πΔft of the above Equation (1) has a minimum value, thestrength of the Δf Hz frequency component is minimized.

Therefore, according to the method of generating the OTDM/OCDMtransmission signal, in the Δf signal component extracting step, the2-OTDM/OCDM signal 45-2 is modulated by a modulation signal 71 having afrequency (F−Δf) Hz, and the strength of the Δf Hz signal component inthe 2-OTDM/OCDM signal 45-2 modulated by the modulation signal 71 isdetected.

Referring to FIG. 6, a relationship between the time delay amount Δtadded to the second encoded transmission signal 39 by the opticaldelayer 40 and the strength of the Δf Hz frequency component in theelectrical modulation OTDM/OCDM signal 49 will be described. FIG. 6 is adiagram illustrating a relationship between the time delay amount Δt andthe strength of the signal component that is included in the electricalmodulation OTDM/OCDM signal 49 having a frequency of Δf Hz. In FIG. 6, ahorizontal axis indicates the time delay amount Δt in a ps unit usingscale marks and a vertical axis indicates the strength of the Δf Hzsignal frequency component in a dB unit using scale marks.

In FIG. 6, the relationship between the time delay amount Δt and thestrength of the signal component where a frequency is Δf Hz is a resultthat is obtained by performing an experiment, under the condition of FHz=9.95328 GHz and Δf=250 MHz (=0.25 GHz). If the time delay amount Δtis set as 21 ps, the strength of the signal component that is includedin the electrical modulation OTDM/OCDM signal 49 and has a frequency ofΔf Hz has a minimum value (refer to (1) of FIG. 6).

The value of Δf Hz that is a frequency output from the oscillator 64 maybe set in a range where the frequency may be easily observed using thespectrum analyzer, which is commercially available. As described above,if the value of Δf Hz is set as Δf=250 MHz (=0.25 GHz), the frequencymay be easily observed using the spectrum analyzer, which iscommercially available.

Referring to FIGS. 7A to 7D, the time waveforms of the 2-OTDM/OCDMsignal 43 for which the time delay amounts Δt are set as 21 ps, 48 ps,71 ps, and 94 ps (illustrated by (1), (2), (3), and (4), respectively,in FIG. 6) will be described. FIGS. 7A to 7D are diagrams illustratingthe time waveforms of the 2-OTDM/OCDM signal 43 when the time delayamounts Δt are set as 21 ps, 48 ps, 71 ps, and 94 ps, respectively. InFIGS. 7A to 7D, the chip pulse groups of the first and second channelsare denoted by “ch1” and “ch2”, respectively, to be easily viewed.

As described above, when the time delay amount Δt is 21 ps, the strengthof the component that is included in the electrical modulation OTDM/OCDMsignal 49 and has a frequency of Δf Hz has a minimum value. That is, inthis case, the time slots are equivalently allocated in the first andsecond channels, as apparent from FIG. 7A. From FIGS. 7B to 7D where thetime delay amount Δt is out of 21 ps, it may be seen that the time slotsare not equivalently allocated in the first and second channels.

Accordingly, in the case of the 2-channel OTDM/OCDM transmission signalgenerating apparatus, if the time delay is applied to the second encodedtransmission signal 39 such that the time delay amount Δt becomes Δt=21ps, the encoded transmission signals of the first and second channelsare arranged at an equivalent interval on the time axis.

In the case of the 2^(N)-channel OTDM/OCDM transmission signalgenerating apparatus, if the time delay amounts (1/2^(N-1))×(21 ps),(2/2^(N-1))×(21 ps), (3/2^(N-1))×(21 ps), and {(2^(N-1))/2^(N-1)}×(21ps) equal to the integral multiple of the time delay amount1/2^(N-1)×(21 ps) are sequentially applied to the optical delayers ofthe second to 2^(N)-th encoded transmission signal generators 120-2 to120-2 ^(N), the encoded transmission signals 121-1 to 121-2 ^(N), theencoded transmission signals 121-1 to 121-2 ^(N) that are output fromthe first to 2^(N)-th encoded transmission signal generators 120-1 to120-2 ^(N) are arranged at an equidistant interval on the time axis.

In this case, an example of a basic method to calculate the time delaysapplied to the encoded transmission signals of the first to M-thchannels on the basis of the time delay amount Δt calculated in the timedelay amount determining step, when optical pulse signals of M channels(M is an integer of 2 or more) are encoded and multiplexed, isillustrated.

When the optical pulse signals of the M channels are encoded andmultiplexed, all of the encoded transmission signals of the first toM-th channels need to be inserted into the time slot of 2Δt. That is,the encoded transmission signals of the first to M-th channels may bearranged at an equidistant interval of 1/M of 2Δt on the time axis.Accordingly, in order to adjust the encoded transmission signals outputfrom the first to M-th encoded transmission signal generators to bearranged at an equidistant interval on the time axis, the time delayamounts 2Δt/M, 2×(2Δt/M), 3×(2Δt/M), . . . , and (M−1)×(2Δt/M) equal tothe integral multiple of the time delay amount 2Δt/M determined on thebasis of the above-described time delay amount Δt may be sequentiallyset to the optical delayers that are included in the second to M-thencoded transmission signal generators.

When M is given as M=2^(N), as described above, the time delay amounts(1/2^(N-1))Δt, (2/2^(N-1))Δt, (3/2^(N-1))Δt, . . . , and{(2^(N)−1)/2^(N-1)}Δt equal to the integral multiple of the time delayamount 2Δt/2^(N)=Δt/2^(N-1) determined on the basis of the time delayamount Δt calculated in the time delay amount calculating step may besequentially set to the optical delayers that are included in the secondto 2^(N)-th encoded transmission signal generators.

When the optical pulse signals of the M channels whose bit rate is Fbit/s are encoded and multiplexed, in addition to inserting of all ofthe encoded transmission signals of the first to M-th channels in thetime slot of 2Δt, the time slot St may need to be separately secured inthe time slot of 2Δt. In this case, St is a real number that satisfiesthe condition 0<δt<2Δt. However, in actuality, δt is generally set to avalue sufficiently smaller than 2Δt.

In this case, the encoded transmission signals that are output from thefirst to M-th encoded transmission signal generators are adjusted to bearranged at an equivalent interval on the time axis, in the remainingtime slot other than the time slot δt. That is, in this case, (2Δt−δt)/Mis used as the time delay amount that is determined on the basis of theabove-described time delay amount Δt.

The time delay amounts (2Δt−δt)/M, 2×(2Δt−δt)/M, 3×(2Δt−δt)/M, . . . ,and (M−1)×(2Δt−δt)/M equal to the integral multiple of (2Δt−δt)/M may besequentially set to the optical delayers that are included in the secondto M-th encoded transmission signal generators, respectively.

1. A method of generating time-division multiplexed encoded transmissionsignals, comprising: encoding optical pulse signals for each of aplurality of multiplexed channels whose bit rate is F bit/s andgenerating a transmission signal for each channel; performing timedivision multiplexing on first and second transmission signals selectedfrom the transmission signals of the plurality of multiplexed channelsand generating a 2-channel multiplexed signal; modulating themultiplexed signal with a modulation signal having a frequency of (F−Δf)Hz, where Δf is defined as a real number satisfying the condition Δf>0and F is defined as a real number satisfying the condition F>Δf;detecting a strength of a Δf Hz frequency component of the multiplexedsignal modulated by the modulation signal; changing a time delay amountof the second transmission signal with respect to the first transmissionsignal, and determining a time delay amount at which a strength of theΔf Hz frequency component is minimized; and applying a time delay to thetransmission signal of each of the multiplexed channels on the basis ofthe determined time delay amount, and adjusting the transmission signalsof the individual channels such that they are arranged at equidistantintervals on a time axis.
 2. A method of generating time-divisionmultiplexed encoded transmission signals, comprising: encoding anoptical pulse signal of a first channel whose bit rate is F bit/s andgenerating and outputting a first transmission signal; encoding anoptical pulse signal of a second channel whose bit rate is F bit/s andgenerating and outputting a second transmission signal; inputting thesecond transmission signal and applying a time delay to the secondtransmission signal; performing time division multiplexing on the firsttransmission signal and the second transmission signal to which the timedelay has been applied, and generating a 2-channel multiplexed signal;modulating the multiplexed signal with a modulation signal having afrequency (F−Δf) Hz, where Δf is defined as a real number satisfying thecondition Δf>0 and F is defined as a real number satisfying thecondition F>Δt and generating a modulated multiplexed signal; convertingthe modulated multiplexed signal into an modulated multiplexedelectrical signal and outputting the modulated multiplexed electricalsignal; changing the time delay amount applied to the secondtransmission signal, and determining a time delay amount at which thestrength of Δf Hz frequency component of the modulated multiplexedelectrical signal is minimized; and setting the time delay of the secondencoded transmission signal on the basis of the determined time delayamount, and adjusting the first and second encoded transmission signalssuch that they are arranged at equidistant intervals on a time axis. 3.A method of generating time-division multiplexed encoded transmissionsignals, comprising: encoding optical pulse signals of 2^(N) channels (Nis an integer of 1 or more) whose bit rate is F bit/s, and generatingtransmission signals; inputting a second transmission signalcorresponding to one transmission signal of first and secondtransmission signals selected from the transmission signals of the 2^(N)channels, and applying a time delay to the second transmission signal togenerate a delayed second transmission signal; performing time divisionmultiplexing on the first transmission signal and the delayed secondtransmission signal and generating 2-channel multiplexed signal;modulating the multiplexed signal by a modulation signal having afrequency (F−Δf) Hz, when Δf is defined as a real number satisfying thecondition Δf>0 and F is defined as a real number satisfying thecondition F>Δf, and generating a modulated multiplexed signal;converting the modulated multiplexed signal into a modulated multiplexedelectrical signal and outputting the modulated multiplexed electricalsignal; changing the time delay amount applied to the secondtransmission signal, and determining the time delay amount Δt which thestrength of a Δf Hz frequency component of the modulated multiplexedelectrical signal is minimized; and sequentially adding the time delayamounts (1/2^(N-1))Δt, (2/2^(N-1))Δt, (3/2^(N-1))Δt, . . . , and{(2^(N)−1)/2^(N-1)}Δt which are integral multiples of the time delayamount Δt/2^(N-1) determined on the basis of the determined time delayamount Δt, to the transmission signals of the second to 2^(N)-thchannels, generating delayed second to 2^(N)-th transmission signals,and adjusting the transmission signals of the individual channels suchthat they are arranged at equidistant intervals on a time axis; andperforming time division multiplexing on the transmission signal of thefirst channel and the delayed second to 2^(N)-th transmission signals towhich the time delay amounts are respectively added, and generating amultiplexed transmission signal.
 4. An apparatus for generatingtime-division multiplexed encoded transmission signals, comprising:encoded transmission signal generators that encode optical pulse signalswhose bit rate is F bit/s and output generated transmission signals, thenumber of the encoded transmission signal generators corresponding tothe number of a plurality of multiplexed channels; an opticalmultiplexer that performs time division multiplexing on first and secondtransmission signals output from two encoded transmission signalgenerators selected from the encoded transmission signal generators, andgenerates a 2-channel multiplexed signal; a spectrum analyzer thatmodulates the multiplex signal by a modulation signal having a frequency(F−Δf) Hz, where Δf is defined as a real number satisfying the conditionΔf>0 and F is defined as a real number satisfying the condition F>Δf,and detects a strength of a Δf Hz frequency component in the multiplexedsignal modulated by the modulation signal; and a optical delay amountcontroller that changes the time delay amount of the second transmissionsignal with respect to the first transmission signal, determines thetime delay amount Δt which the strength of the Δf Hz frequency componentis minimized, sets the time delay amount of the transmission signal ofeach of the multiplexed channels on the basis of the determined timedelay amount Δt, and adjusts the transmission signals of the individualchannels such that they are arranged at equidistant intervals on a timeaxis.
 5. An apparatus for generating time-division multiplexed encodedtransmission signals, comprising: first and second transmission signalgenerators that encode optical pulse signals whose bit rate is F bit/s,and generate and output generated transmission signals; an opticaldelayer disposed in the second transmission signal generator thatapplies a time delay to a transmission signal output from the secondtransmission signal generator; an optical multiplexer that performs timedivision multiplexing on the first transmission signal and the secondtransmission signal to which the time delay has been applied, andoutputs a 2-channel multiplexed signal; an optical branching device thatbranches the multiplexed signal into a multiplexed signal fortransmission and a multiplexed signal for monitoring; an opticalmodulator that receives the multiplex signal for monitoring, modulatesthe multiplexed signal for monitoring with a modulation signal whosefrequency is (F−Δf) Hz, where Δf is defined as a real number satisfyingthe condition Δf>0 and F is defined as a real number satisfying thecondition F>Δf, and generates a modulated multiplexed signal; aphotoelectric converter that receives the modulated multiplexed signal,converts the modulated multiplexed signal into a modulated multiplexedelectrical signal, and outputs the modulated multiplexed electricalsignal; a spectrum analyzer that detects the strength of a Δf Hzfrequency component of the modulated multiplex electrical signal; and anoptical delay amount controller that sets the optical delayer to thetime delay amount to apply a time delay at which the strength of a Δf Hzfrequency component is minimized to the second transmission signal, andadjusts the transmission signals of the multiplexed channels such thatthey are arranged at equidistant intervals on a time axis.
 6. Anapparatus for generating time-division multiplexed encoded transmissionsignals, comprising: first to 2^(N)-th (N is an integer of 1 or more)encoded transmission signal generators that encode optical pulse signalswhose bit rate is F bit/s, and output generated transmission signals;optical delayers that are disposed in the second to 2^(N)-th encodedtransmission signal generators, respectively, to apply time delays tothe transmission signals output from the second to 2^(N)-th encodedtransmission signal generators; an optical multiplexer that performstime division multiplexing on the transmission signals output from thefirst to 2^(N)-th encoded transmission signal generators and generates amultiplexed transmission signal; a sub-optical multiplexer that performstime division multiplexing on first and second transmission signalsoutput from first and second encoded transmission signal generatorsselected from the first to 2^(N)-th encoded transmission signalgenerators and generates a 2-channel multiplexed signal; an opticalmodulator that modulates the multiplexed signal by a modulation signalwhose frequency is (F−Δf) Hz, when Δf is defined as a real numbersatisfying the condition Δf>0 and F is defined as a real numbersatisfying the condition F>Δf, and generates a modulated multiplexedsignal; a spectrum analyzer that detects a strength of a Δf Hz frequencycomponent of a multiplexed signal, which changes according to a changein the time delay amount applied to the second encoded transmissionsignal; and an optical delay amount controller that changes the timedelay amount applied to the second encoded transmission signal,determines a time delay amount Δt which the strength of a Δf Hzfrequency component of a modulated multiplexed electrical signal isminimized, sequentially sets the time delay amounts (1/2^(N-1))Δt,(2/2^(N-1))Δt, (3/2^(N-1))Δt, . . . , and {(2^(N)−1)/2^(N-1)}Δt whichare integral multiples of the time delay amount Δt/2^(N-1) determined onthe basis of the determined time delay amount Δt, to the opticaldelayers of the second to 2^(N)-th encoded transmission signalgenerators, and adjusts the transmission signals output from the firstto 2^(N)-th encoded transmission signal generators such that they arearranged at equidistant intervals on a time axis.